Many integrated circuit functions require precise voltage and current references. For example, an analog-to-digital converter typically requires a precise voltage reference to establish and quantize an analog input voltage range. In another example, many analog filters, such as transconductance-capacitance (g.sub.m C) filters, have filter gain and rolloff frequency characteristics that depend upon their bias currents. A precise current reference is useful for generating accurate bias currents in such filters and other circuits.
Many battery powered electronics applications, including implantable cardiac rhythm management systems, need current and temperature compensated voltage reference circuits that operate at low power supply voltages and power consumption. There is a critical need for circuits, including reference circuits, that operate at low power supply voltages and draw less current from the low voltage power supply in order to increase battery longevity. There is also a need for such reference circuits in which the value of the reference voltage and reference current are capable of fine adjustment. There is a further need for such reference circuits in which the resulting reference voltage is temperature compensated, i.e. the sensitivity of the reference voltage to temperature variations is reduced. Moreover, the internal impedance of the battery results in variations of the terminal voltage. When significant current is being drawn from the battery, the terminal voltage of the battery can droop significantly. There is a need for a reference circuit that can accommodate such power supply voltage variations.